viernes, 8 de junio de 2012

STEARMAN MODEL 75: FLIGHT IN ITS PUREST FORM

Text and Indicated Photos: José Manuel Serrano EsparzaSPANISH Introduced in 1936, the radial piston engined biplane Boeing Stearman Model 75 is considered one of the most genuine representatives of the Golden Era of Aviation, excelling because of its manoeuvrability, sturdiness and reliability conferring it a remarkable stability during flight.

Besides, its flight qualities focused on the most essential sides, with a unique design fulfilling both the U.S Army and U.S navy allowed it throughout a lot of decades into a great military trainer aircraft with an exceedingly good controls response, which was a key factor in the basic instruction of hundreds of thousnads of pilots all over the world, who took advantage of its easy handling, its tolerance to hard use and the performance consistency of its usual Continental R-670-5 powerplant of the U.S Army version (also known as PT-17) 220 hp powerplant or the Lycoming R-680-5 engine of the U.S Navy one (also known as N2S).

On the other hand, it was easy to repair in case of breakdown and his maintenance was extremely easy.

The Stearman 75 also stands out because of its majestic flight and the fairly characteristic sound of its ´round engine´, and after the Second World War thousands of them were used as civil planes in fumigation labours - thanks to its excellent capability for slow flight at low altitude - and in aerobatics competitions, being often provided with different engines sporting more power than the standard 220 hp Continental R-670-5 power plant, above all with Pratt & Whitney Wasp Junior R-985 delivering 450 h.p.

ORIGIN
The Boeing Stearman Model 75 is the final offspring of several military training aircraft which had their dawn with the X-70 biplane (inspired by the 1931 Stearman Model 6 ´Cloudboy´ featuring a 165 hp Wright J-6 Whirlwind-5), equippped with a 210 hp Lycoming R-680 and designed by Harold Zipp and Jack Clark, which was firstly made by Stearman Aircraft Corporation for the civil market, and whose maiden flight took place on January 1, 1934, after a 60 day development.

Since then and till Second World War, top priority was reaching an evolutive pinnacle resulting in a two-place biplane trainer boasting great building construction, outstanding reliability and tolerance with novice pilots and an engine with the best possible production cost/consistency of results.

Likewise, the X-70 was manufactured with such an airframe that it could endure load factors superior to the maximum ones which could be expected during traning flights, so optimizing the safety of future pilots who would have to acquire their first basic flight skills with it.

Therefore, in mid 1934, both the U.S Army and U.S Navy carried out tests with the Stearman X-70 prototype which brought about its massive adoption by the U.S Navy and a brief delay in its supply for the U.S Army, which chose to wait for the introduction of the Stearman Model 75 in 1936.

This way, the U.S Navy was delivered its first order of Stearmans (still very similar to the X-70 prototype), called NS-1 (also known as Model 73) and equipped with the somehow a bit obsolete Wright R-790-8, though it showed its noteworthy performance as a primary trainer, so in early 1935, the U.S Navy decided to buy some thousand units of the N2S, a model already advanced enough in comparison to the X-70 starting prototype, and which was built in five subvariants, each one being set up with a didferent engine.

And it was in 1936 when the appearance of the Stearman Model 75 itself happened, with its first version: the PT-13 model equipped with a Lycoming R-680-5 215 hp engine, which was adopted in 1936 by the U.S Army Corps (that bought 26 units) and the U.S navy (models N2S-2 and N2S-5), which was replaced in 1940 by its second version, the PT-17 model, provided with a Continental R-670-5 220 hp engine, that was taken over by the U.S Army Corps and the U.S Navy (models N2S-1, N2S-3 and N2S-4).

Within time, vast majority of American pilots who took part in Second World War would acquire their flight basic expertise on board of this historical biplane.

GREAT SUCCESS AND INTERNATIONAL SPREADING
Thanks to its piloting easiness, its great ruggedness and the relatively low cost of both its engine and airframe, the Boeing Stearman 75 reaped a very significant quantity of sales, and the Stearman Aircraft Company at Wichita (Kansas), a subsidiary of Being, worked full steam ahead between mid thirties and 1944, making a total of 8,584 units, which is the absolute record as to biplane airships in the history of American aviation.

Evidently, the entering of United States into Second World War from 1941 was the event greatly catapulting the figure of orders of Stearman 75s, both by the U.S Army and the U.S Navy, which needed massive quantities of aircraft for military basic training, in the same way as canada (where it was exported and received the Kaydett denomination), in such a way that thousands and thousands of units were bought for the basic learning of future combat pilots.

But the inherent qualities of the Stearman Model 75 made that later and up to nowadays its starting role as military training aeroplane was greatly stretched, developing an operative life that after the Second World War was essentially civil, becoming a very beloved aircraft oozing charisma and highly appreciated by the enthusiasts of aviation and the beautiful radial piston engined classic planes.

The key factor in its postwar spreading, already mostly in the civil sphere, was its great versatility, since as well as being the basic two-seat military trainer both in the U.S Army and U.S Navy for almost ten years (many dozen thousands of pilots learned their fundamental flight skills with it before making the transition to most powerful advanced training planes like the Texan T-6 and the subsequent flying of fighters like the P-51 Mustang, F4U Corsair, etc, and bombers like the B-17 and B-29), also turned into a top-notch choice as a flight instrument trainer, in tasks of fumigation, reconnaissance missions, sowing of rice fields, aerobatics, aerial photography, spped contests, appearance in famous Hollywood movies, and even performing the role of light attack in some air forces of Latin American countries like Brasil, Venezuela, Argentina and Cuba, together with Philippines, equipped with more powerful engines like the Wright Cyclone 420 hp, Wright R760 320 hp, the Pratt & Whitney Wasp 320 hp, the Lycoming R-680-13 300 hp and the Wright Whirlwind 235 hp, without forgetting abundant quantities of Starman 75s which were exported to China.

In the same way, from scratch, it was acquired by many fans of sporting aviation in the five continents, who found in it the ideal plane to enjoy flight in its purest form, becoming a full-fledged classic in the History of Aviation, with its unmistakable appearance and its slow yet stately flight.

On the other hand, it mustn´t be forgotten that the Boeing Stearman 75 was the first aircraft used to land on the deck of an aircraft-carrier, for it was able to take off and land on very short stretches of space and on every kind of surface, due to its great sturdiness, a fact that was instrumental in its expansion in the scope of sporting aviation and its frequent presence in all kind of aerodromes and airports, from the most sophisticated to those ones located on remote zones of difficult access, even in jungle areas where the dimensions of the airstrips are often very reduced.

A HIGHLY RELIABLE ENGINE

Photo: José Manuel Serrano Esparza

The Stearman 75 is usually equipped with a Continental R-670-5 220 hp at 2,175 rpm air-cooled engine featuring seven cylinders in four cycles, with a dry weight of 211 kg and 10.94 litres of displacement.

It has a length of 79.2 cm and a diameter of 1.079 m, with a bore x stroke of 130.2 x 117.5 mm.

It also sports a Stromberg carburettor, together with an inlet and exhaust valve per cylinder, its ratio of compression being 5:4:1.

Photo: José Manuel Serrano Esparza

Photo: José Manuel Serrano Esparza

From a diachronic viewpoint, the Stearman 75 has proved its full adaptability to all kinds of engines, from the basic PT-17 and N2S3 featuring a Continental R-670-5 power 220 hp and 9 cylinder power plant to the A75L300 reconverted for civil use with a Lycoming R-680-13 300 hp engine, without forgetting the most powerful versions like the Super Stearman IB75 with a Pratt & Whitney R-985 Wasp Jr 450 hp and two extra ailerons in the upper wing.

Needless to say that each version of the Stearman 75 with its various engines and powers, has a different flight dynamics and engine sound, which adds even more allure to the biography of this mythical biplane, which goes on currently alive and kicking in many aerial exhibitions all over the world and whose watching raises the appraisal and enthusiasm of everybody having the chance of seeing it in action.

EXCEPTIONAL BUILDING CONSTRUCTION
The Boeing Stearman 75 is made with unusual strength of components for mid thirties, the time of its introduction, being prominent because of its mixed construction,

with specially resistant wooden wings covered with fabric, while the fuselage is made up by an exceedingly hard tubular structure of very thick welded stainless steel also covered with fabric.

On the other hand, the fixed landing gear likewise sports a huge sturdiness,

with legs which are part of a rigid one piece system which is bolted to the lower area of the airship,

and with wheels integrated in assemblies axle/support sliding upwards and downwards in the lower end of the landing gear legs.

Besides, the rear wheel shock absorber and the support set have a bigger weight than the steering gear of many medium size light aeroplanes.

And the main units of cantilevers feature high quality oleo pneumatic struts.

Bearing it all in mind, Boeing deserves accolades for having been able to build a lot of thousands of Stearmans 75 with a production cost of only 9,500 dollars of the time.

On the other hand, the Stearman 75 is a huge size aircraft in comparison to the De Havilland Tiger Moth, the SVA Stamp and the Bücker Jungmann, which were the most popular biplane trainers during Second World War, and its empty weight of 1,936 pounds - 878 kg - ( with a maximum possible weight at takeoff of 2717 pounds - 1232 kg -) is roughly equivalent to the one of Bücker Jungmann plus any of the other two airplanes together.

It must also be taken into account the significant fact that the Stearman 75 cockpit is much larger and ample, so the pilot has a greater movement freedom at avery moment, and the big diecast aluminum pedals governing the rudder sport an unmatched strength and durability in its plane category, easily adjusted and hanging from a solid structure.

On its turn, the control stick sports a much larger size than usual in this sort of aircraft, which enables its continuous use without a hitch for decades, a trait perfectly synergizing with the chance of selecting nothing less than eleven different seating positions so as to achieve maximum comfort for the pilot during flight.

OUTSTANDING PERFORMANCE AS AN AEROBATIC AIRCRAFT
When analysing the aerobatic ability of the Stearman 75, two basic types should be distinguished:

a) In its basic production configuration with a Continental R-670-5 220 hp, Lycoming-R 220 hp and some power plants Wright and Jacobs with powers between 215 and 225 hp, the climbing rate of the Stearman Model 75 is relatively limited by its remarkable maximum weight of 2717 pounds - 1232 kg - and its much larger size than the trainers of its time (De Havilland Tiger Moth, Polikarpov Po-2, Arado Ar 66, Naval Aircraft Factory N3N Canary, Fleet Finch, Gotha Go 145, Bucker Bü 131 Jungmann, PWS-26, Focke-Wulf Fw 144, etc), so the Stearmans 75 usually equipped with any of those engines aren´t often used in aerobatics competitions.

In spite of it, the Stearmans 75 featuring any of those basic engines boast a remarkable aerobatic ability for a basic training plane created in mid thirties, and thanks to its exceedingly tough airframe, it is able to endure 12Gs positive and 8Gs negative.

It shouldn´t be forgotten that during thirties and forties, both the U.S Army, the U.S Navy and the Canadian Air Force used the Stearman 75 with its two besic 220-225 hp engines as a primary trainer and aerobatic plane.

b) In its configuration as an aerobatic aircraft featuring a constant speed propeller (mostly Hamilton Standard 2B/20) and equipped with engines Lycoming R-680-13 300 hp, Wright J-6-9 Whirlwind 300 hp, Jacobs R-755-A2 300 hp, Pratt & Whitney Wasp Junior R-985 450 hp and even more powerful power plants like the Osmoh 600 hp and the Pratt & Whitney R-1340 Wasp 600 hp, often being provided with two extra ailerons under the upper wing, the Stearman 75 becomes a high performance aerobatic aeroplane, with the added benefit that each version is unique and has its own flight dynamics.

The installing of all of these very powerful engines boasting greater performance and weight than the basic 215-225 hp power plants of the Stearman 75 is feasible thanks to the very sturdy structure of the aircraft, full of pipe size reinforced aluminum tubes, which provides outstanding resistance and versatility to the airship, enabling to make spins, inner loops, quick rolls under 108.7 mph (175 km/h), slow rolls under 126.78 mph (204 km/h), Immelmann turns, inverted flight, inverted spins, etc.

EXCELLENT PLANE IN FUMIGATION LABOURS
Throughout many consecutive decades after Second World War, the Stearman 75 became the backbone of aircraft fleets used in agriculture, above all in United States and Canada, specializing in fumigation and seed-planting and mainly equipped with the high efficiency Pratt & Whitney R-985 Wasp Jr 450 hp engine, which delivered double the power than the original Continental R-670-5 220 hp, since the pilots devoted to these toils in rural areas needed a superior response control on flying at a height of five feet, since besides, this biplane doesn´t allow to see utterly around it, and at so low flight, any unexpected obstruction of sight could be fatal.

A Stearman 75 aircraft intended for fumigation missions and appearing in the 1959 movie North by Northwest, directed by Alfred Hitchcock and featuring Cary Grant, Eva Marie Saint and James Mason as main characters made this already highly famous plane even more popular worldwide.